The long-term goal of the proposed research is to understand with atomic-resolution detail the basis for viral fusion protein-induced membrane fusion. Fusion of viral and target cell membranes is a key step in infection for many viruses important in disease and a detailed understanding of fusion mechanisms should aid development of anti-viral therapeutics whose mode of action is fusion inhibition. The proposed research focuses on the gp41 and the hemagglutinin HA2 fusion proteins of the human immunodeficiency virus (HIV) and the influenza virus (IFV), respectively, because of the significance of the diseases caused by these viruses and because these proteins serve as prototypes for other class I viral fusion proteins. There is particular emphasis on the "fusion peptide" (FP) domains of the fusion proteins, which represent approximately 20-residue apolar regions at the N-termini of the proteins. Numerous biochemical and biophysical studies have shown that the FP plays a key role in fusion and that chemically synthesized peptides with the FP sequence can serve as useful model systems to understand some aspects of viral/target cell fusion. The long-term objectives will be achieved with four specific aims. Two of the aims focus on solid-state nuclear magnetic resonance (SSNMR) structural analysis of chemically cross-linked dimeric and trimeric HIV FPs in membranes and additional liquid-state NMR studies of these peptides in detergent micelles. The oligomeric topology of these C-terminal cross-linked peptides reflects the expected topology of FPs in the HIV gp41 fusion protein. The third aim focuses on: (a) determination of the insertion orientation of FPs in membranes using samples in which the membranes are oriented between stacked glass plates; and (b) probing FP insertion depths using SSNMR distance measurements between 13C nuclei in the FP and 31P and 19F nuclei in the lipid. The fourth specific aim focuses on SSNMR structural and insertion depth measurements for large HIV and IFV fusion protein domains which contain FPs.